I can't figure this out, I have been googling a lot and can't find it. In Musicbee there is a "keywords" field. In Mp3tag program if I define a tag called "KEYWORDS" then it will show up in musicbee under keywords.
However I'm trying to figure out what the frame is called to write it in my code. Does anyone know what it is?
ID3v2 does not know a dedicated frame for that. As per v2.4.0, chapter 4.2.6 use a TXXX frame with the the appropriate description.
Inofficially the TKWD is supported by multiple taggers and software as "Podcast Keywords" - most likely iTunes started using it on its own - see Mp3tag and mutagen and ExifTool.
In both variants you risk mumbling to yourself, because not every software expects all possibilities. Likewise you should be flexible enough yourself to expect other software to write it again in a different way.